The Spirit is the same yesterday, today, and forever. So it should not surprise us that the doctrine of the joy of missionary work was given to Ammon long before it was revealed to Joseph Smith. The missionary work of the sons of Mosiah may demonstrate the quintessential practical example of the following scripture:
'And if it so be that you should labor all your days in crying repentance unto this people, and bring, save it be one soul unto me, how great shall be your joy with him in the kingdom of my Father!
And now, if your joy will be great with one soul that you have brought unto me into the kingdom of my Father, how great will be your joy if you should bring many souls unto me!' (DC 18:15-16)